Article pubs.acs.org/IECR
Calculation of Standard Enthalpy of Formation of Hexyl Acrylate from Chemical Equilibrium Emine Sert* Chemical Engineering Department, Ege University, Bornova, Izmir, Turkey ABSTRACT: Chemical equilibrium was investigated for the quaternary system consisting of hexanol, acrylic acid, hexyl acrylate, and water. The equilibrium mole fractions, equilibrium conversions, and equilibrium constants at 338, 348, and 358 K were calculated. The activity coefficients were estimated using UNIQUAC method to account for the nonideal thermodynamic behavior. The heat of formation of hexyl acrylate was found to be −449.59 kJ/mol from equilibrium experiments. Also, three estimation methods for the calculation of the enthalpy of formation of hexyl acrylate are determined and compared with the experimental results. All methods are based on group or bond contribution. The Joback method was found to be more accurate in predicting the actual values of the enthalpy of formation of hexyl acrylate.
1. INTRODUCTION Acrylic esters are versatile monomers and widely used for the production of coatings, adhesives, textiles, and plastics.1 Large volumes of acrylic emulsion polymers are used as binders for fiberfill and nonwoven fabrics, textile bonding or laminating, flocking, back coating, and pigment printing binders. 2 Particularly, hexyl acrylate is used in the production of inks, paints, coatings, cosmetics, binders, and adhesives. Current production of hexyl acrylate is the esterification of acrylic acid with hexanol catalyzed by acidic catalyst. Esterification reactions are characterized by thermodynamic limitations on conversion. Higher ester yields can be obtained by shifting the reaction toward products formation by hybrid processes such as reactive distillation and pervaporation-aided reactor instead of using a large excess of one of the reactants, usually the alcohol.3 In both reactive distillation and pervaporation-aided reactor, esterification systems were identified by using phase equilibrium and reaction kinetics.4 These processes allow overcoming equilibrium limitations. In the past decade, the pervaporation-esterification hybrid process has become an alternative to the usual reaction separation processes.5,6 The first step in all separation−reaction hybrid systems is an analysis of chemical equilibrium. There are important experimental studies on the chemical equilibrium of esterification reaction in the literature.7−9 Physical and thermodynamic properties data of compounds are needed in the design and operation of industrial chemical processes. The standard enthalpy of formation, ΔH°f is an important fundamental physical property of compounds which is defined as change of enthalpy that accompanies the formation of 1 mol of compound in its standard state from its constituent elements in their standard state. The standard enthalpy change of formation is used in thermochemistry to find the standard enthalpy change of reaction.10 Although there are some equilibrium studies for esterification reaction in literature, no study has been performed on the chemical equilibrium or synthesis for the esterification of acrylic acid with hexanol. In this study, the influence of temperature on the equilibrium constant was investigated in a batch reactor. Equilibrium conversions and equilibrium constants at different © 2013 American Chemical Society
temperatures were found experimentally. Also, heat of formation of hexyl acrylate was calculated from three different methods, namely, Joback, Benson group contribution, and Benson bond contribution methods. Predicted heat of formation was compared with the experimental value for hexyl acrylate.
2. EXPERIMENTAL STUDY Acrylic acid and hexanol were used as reactants and sulfuric acid used as catalyst to decrease the reaction time. The reactor consisted of a two-necked glass flask of 500 mL capacity connected with a condenser to prevent any loss of products. An electrical heater was used to heat the reaction mixture, and the reaction mixture was stirred magnetically using a magnetic stirrer. The temperature of the reaction mixture was controlled using a temperature controller. Acrylic acid, catalyst (3 vol %), and phenothiazine as inhibitor were charged into the reaction vessel. The temperature of the reactor was set to the desired value. After the reaction temperature was reached, hexanol was added into the reactor and the reaction mixture was allowed to reach equilibrium for 24 h. The reaction temperatures studied were 338, 348, and 358 K. Equilibrium mole fractions of hexanol, acrylic acid, water, and hexyl acrylate were calculated to determine the overall equilibrium constant. A Hewlett-Packard 6890GC gas chromatograph was used to analysis of the samples. The HP-FFAP polyethylene glycol connected with flame ionization detector to detect the compounds. Hydrogen was used as carrier gas. Injector, detector, and oven temperatures were 473.15, 503.15, and 423.15 K, respectively. The temperature program of gas chromatographic analysis was given as follows: waiting 3 min at 60 °C; heating from 60 to 180 °C at a rate of 5 °C/min; waiting for 3 min at 180 °C. Received: Revised: Accepted: Published: 2840
November 14, 2012 February 6, 2013 February 7, 2013 February 7, 2013 dx.doi.org/10.1021/ie303131x | Ind. Eng. Chem. Res. 2013, 52, 2840−2843
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3. THEORY Hexyl acrylate (HeAc) is formed by the reaction of n-hexanol (HeOH) and acrylic acid (AcAc). This reaction is a typical acid catalyzed, equilibrium limited esterification: C3H4O2 + C6H13OH ⇄ C9H16O2 + H 2O 3.1. Calculation of Equilibrium Constant for Hexyl Acrylate Synthesis. No information about the effect of temperature on the equilibrium constant for the esterification of acrylic acid with hexanol can be found in the literature. The experimental runs were carried out at temperatures of 338, 348, and 358 K, a molar ratio of acid to alcohol of 1, and a catalyst loading of 3 vol % for the esterification of acrylic acid with hexanol. Experiments were undertaken to determine the equilibrium mole fractions of hexanol, acrylic acid, hexyl acrylate, and water. In all experiments, equimolar amounts of acrylic acid and hexanol were used. The equilibrium constant was calculated experimentally according to the following formula; γ γ x x K = HeAc water HeAc water xAcAcx HeOH γAcAcγHeOH
Figure 1. Logarithm of equilibrium constant versus the reciprocal of the absolute temperature.
to 0.896 because of the exothermicity of the reaction. However, the equilibrium constant in most esterification reaction is a weak function in temperature because of small values of heat of reaction. Iż ci and Altıokka12 found the equilibrium constants to be independent of temperature while studying the esterification of acetic acid with iso-butanol. In fact, for the 2-propyl alcohol/ acetic acid esterification system, Agreda et.al.13 found the equilibrium constant for the esterification of methyl alcohol with acetic acid also to be independent of temperature. In our previous study,11 the equilibrium constants were found to be 1.06, 1.11, 1.20, and 1.27 at temperatures of 333, 338, 343, and 348 K, respectively. 3.2. Calculation of Heat of Formation of Hexyl Acrylate. In order to determine the equilibrium of any chemical reaction, the standard enthalpies of formation must be known for the individual compounds that participate in the reaction. The heat of reaction was calculated from following equation;
where xi is the mole fraction of component i at equilibrium and γi is the activity coefficient of component i calculated by the UNIQUAC method. The equations of UNIQUAC method was given in our previous study11 and surface and volume parameters of hexanol−acrylic acid quaternary system were given in Table 1. Table 1. Volume Parameters and Equations of UNIQUAC Activity Model volume and area parameters hexanol acrylic acid hexyl acrylate water
r
q
5.273 2.646 6.244 0.92
4.748 2.400 5.416 1.42
ln K (T ) = ln K (T0) −
ΔHr ⎛ 1 1⎞ ⎜ − ⎟ R ⎝T T0 ⎠
The heat of reaction can be found as −38.6 kJ/mol from slope of Figure 1. The negative value of heat of reaction is an indication of the equilibrium constant decreased as temperature increased. The reaction enthalpy, ΔHr, is calculated from the standard heat of formation, ΔHf, of the reactants and products:
The equilibrium mole fractions, equilibrium conversions, activity coefficients, and equilibrium constants at different temperatures are listed in Table 2. As shown from Table 2, equilibrium constants decreased as temperature was decreased because of the exothermicity of the reaction. The plot of natural logarithm of equilibrium constant values as a function of the inverse of absolute temperature is shown in Figure 1. The ln K by 1/T gives the equation obtained is shown below:
ΔHr =
∑
(ΔHf ) −
products
⎛ 4643.2 ⎞ − 7.9232⎟ K = exp⎜ ⎝ T ⎠
∑
(ΔHf )
reactants
The standard heat of formation is the enthalpy change upon formation of the material from the elements in their standard states at the temperature T.14 The standard enthalpies of formation of acrylic acid, water, and hexanol are listed in Table 3. The heat of formation of hexyl acrylate was calculated as
Increasing the temperature from 338 to 358 K, the equilibrium conversion of acrylic acid decreased from 0.926
Table 2. Equilibrium Conversions, Mole Fractions, and Equilibrium Constants for the Esterification of Acrylic Acid with Hexanol equilibrium mole fractions
activity coefficients
T (K)
Xeq
HeOH
AcAc
HeAc
water
HeOH
AcAc
HeAc
water
K
338 348 358
0.926 0.919 0.896
0.037 0.0405 0.052
0.037 0.0405 0.052
0.463 0.4595 0.448
0.463 0.4595 0.448
1.456 1.442 1.421
0.736 0.745 0.750
1.678 1.667 1.657
1.287 1.276 1.264
315.7 255.0 146.0
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−449.59 kJ/mol by using heat of reaction and heat of formation values of acrylic acid, hexanol, and water.
Table 4. Bond Types and Coefficients (kJ/mol)15 type
Table 3. Standard Enthalpies and Gibbs Energies of Formation component
ΔHf (kJ mol−1)
ΔGf (kJ mol−1)
hexanol acrylic acid water
−316.54 −336.45 −242.00
−134.47 −286.25 −228.77
abbreviation
contribution
Joback CH2 CH2 COO CH CH< OH O
J1 J2 J3 J4 J5 J6 J7 Benson bond contribution CH B1 CC B2 CO B3 COC B4 COO B5 CdH B6 CdH B7 OH B8 Benson group contribution [C(C)(H)3] G1 [C(C)2(H)2] G2 [C(CO)(C)(H)2] G3 [CO(C)(O)] G4 [O(CO)(C)] G5 [C(O)(H)3] G6 [C(CO)(H)3] G7 [CO(H)(O)] G8 [Cd(C)(H)] G9 [C(Cd)(C)(H)2] G10 [C(C)2(H)2] G11 [O(C)(H)] G12 [C(Cd)2(H)2] G13 [Cd(Cd)(H)] G14 [C(Cd)(H)3] G15 [Cd(H)2] G16 [C(Cd)(CO)(H)2] G17 [C(O)(C)(H)2] G18 [Cd(CO)(H)] G19 [CO(Cd)(O)] G20
3.3. Prediction of Heat of Formation. There are many methods for calculation of ΔHf° in the literature, only three methods are widely used. The Joback method, an extension of the Lydersen method, is one of the most traditional contribution methods. The Benson group method is a more complicated and more accurate method, which considers not only the contribution of each functional group but also the interaction of one group with its neighbors. A simpler but less precise method for estimating thermochemical properties was also proposed by Benson. It is defined as the Benson bond contribution method, thus the type of bonds are first identified and counted and the value of the molecular property is calculated by adding the contribution of all bonds.15 The values assigned for each functional group for these three methods are presented in Table 4. In all methods, n is the number of carbon atoms of the acid chain, db is the number of double bond. For acrylic acid esters, n equals 3 and db equals 1. Joback Contribution Method. ΔHf°(methyl ester, db = 0) = 68.29 + 2J1 + (n − 2)J2 + J3 ΔHf°(methyl ester, db = 1) = ΔHf°(methyl ester, db = 0) + ( −2J2 + 2J4 )db ΔHf°(ethyl ester, db = 1) = ΔHf°(methyl ester, db = 1) + J2
−76.45 −20.64 −337.92 37.97 28.89 −208.04 −247.61 −16.04 11.43 −50.24 −60.29 −211.43 13.40 28.05 −113.04 −41.87 −20.93 −21.77 −147.38 −180.45 −41.87 −41.87 −134.40 35.80 −20.10 −30.14 −158.47 −18.00 28.39 −41.87 26.25 −15.91 −33.91 20.93 −120.16
ΔHf°(methyl ester, db = 0) = G1 + (n − 3)G2 + G3
ΔHf°(hexyl ester, db = 1) = ΔHf°(methyl ester, db = 1)
+ G4 + G5 + G6
+ 5J2
ΔHf°(methyl ester, db = 1) = ΔHf°(methyl ester, db = 0)
Benson Bond Contribution Method.
+ ( −4G2 − 2G9 + 2G10)db
ΔHf°(methyl ester, db = 0) = (2n + 2)B1 + (n − 2)B2
ΔHf°(ethyl ester, db = 1) = ΔHf°(methyl ester, db = 1)
+ B3 + B4 + B5
− G6 + G1 + G18
ΔHf°(methyl ester, db = 1) = ΔHf°(methyl ester, db = 0) + ( −2B1 − 2B2 + 2B6 + 2B7 )db
ΔHf°(hexyl ester, db = 1) = ΔHf°(methyl ester, db = 1)
ΔHf°(ethyl ester, db = 1) = ΔHf°(methyl ester, db = 1)
− 5G6 + 5G1 + 5G18
+ 2B1 + B2
All of the estimation methods explained above give the estimated heat of formation in the gas phase. Gas phase heat of formation can be adjusted to the liquid phase by subtracting the enthalpy of vaporization, ΔHvap. ΔHvap was estimated as 40.57 kJ/mol according to a prediction method explained in the literature.14
ΔHf°(hexyl ester, db = 1) = ΔHf°(methyl ester, db = 1) + 5(2B1 + B2 )
Benson Group Contribution Method. 2842
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(4) Wyczesany, A. Chemical equilibrium constants in esterification of acetic acid with C1−C5 alcohols in the liquid phase. Chem. Eng. Process 2009, 30, 243−265. (5) Delgado, P.; Sanz, M. T.; Beltran, S.; Nunez, L. A. Ethyl Lactate Production via Esterification of Lactic Acid with Ethanol Combined with Pervaporation. Chem. Eng. J. 2010, 165, 693−700. (6) Hasanoğlu, A.; Salt, Y.; Keleşer, S.; Dinçer, S. The Esterification of Acetic Acid with Ethanol in a Pervaporation Membrane Reactor. Desalination 2009, 245, 662−669. (7) Ali, S.; Merchant, S. Q. Kinetic Study of Dowex 50 Wx8Catalyzed Esterification and Hydrolysis of Benzyl Acetate. Ind. Eng. Chem. Res. 2009, 48 (5), 2519−2532. (8) Ali, S. H.; Merchant, S. Q. Kinetics of the Esterification of Acetic Acid with 2-Propanol: Impact of Different Acidic Cation Exchange Resins on Reaction Mechanism. Int. J. Chem. Kinet. 2006, 38, 593− 612. (9) Ali, S. H.; Tarakmah, A.; Merchant, S. Q.; Al-Sahhaf, T. Synthesis of esters: Development of the rate expression for the Dowex 50Wx8− 400 catalyzed esterification of propionic acid with 1-propanol. Chem. Eng. Sci. 2007, 62, 3197−3217. (10) Vatani, A.; Mehrpooya, M.; Gharagheizi, F. Prediction of Standard Enthalpy of Formation by a QSPR Model. Int. J. Mol. Sci. 2007, 8, 407−432. (11) Sert, E.; Atalay, F. S. Determination of Adsorption and Kinetic Parameters for Transesterification of Methyl Acetate with Hexanol Catalyzed by Ion Exchange Resin. Ind. Eng. Chem. Res. 2012, 51, 6350−6355. (12) Izci, A.; Bodur, F. Liquid Phase Esterification of Acetic Acid with iso-Butanol Catalyzed by Ion Exchange Resins. React. Func. Polym. 2007, 67 (12), 1458−1464. (13) Agreda, V. H.; Partin, L. R.; Heise, W. H. High-Purity Methyl Acetate via Reactive Distillation. Chem. Eng. Prog. 1990, 86 (2), 40− 46. (14) Weisenburger, G. A.; Barnhart, R. W.; Clark, J. D.; Dale, D. J.; Hawksworth, M.; Higginson, P. D.; Donald, Y. K.; Knoechel, D. J.; Moon, B. S.; Shaw, S. M.; Taber, G. P.; Tickner, D. L. Determination of Reaction Heat: A Comparison of Measurement and Estimation Techniques. Org. Process Res. Dev. 2007, 11, 1112−1125. (15) Lapuerta, M.; Fernández, J. R.; Oliva, F. Determination of enthalpy of formation of methyl and ethyl esters of fatty acids. Chem. Phys. Lipids 2010, 163, 172−181. (16) Poling, B. E.; Prausnitz, J. M.; O’Connell, J. P. The Properties of Gases and Liquids, 5th ed.; McGraw-Hill: New York, 2000.
The results for standard enthalpy of formation of hexyl acrylate obtained by Joback, Benson bond contribution, and Benson group contribution methods are presented in Table 5. Table 5. Standard Enthalpy of Formation of Hexyl Acrylate heat of formation of hexyl acrylate, ΔHf (kJ/mol) Experimental Estimation
Joback Benson bond Benson group
−449.59 −469.72 −490.55 −528.34
As shown from Table 5, the Joback method provides estimated values closer to the experimental results, and also the Joback method gives higher heat of formation value for hexyl acrylate. In the study of Poling et al.,16 the Joback method was applied in order to calculate the standard enthalpy of formation for more than 300 compounds, reporting a mean deviation around 10% between them and experimental data. In this study, deviation between heat of formation of hexyl acrylate according to the Joback method and experimental data was found as 4.47%.
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CONCLUSION The chemical equilibrium of a quaternary system consisting of hexanol, acrylic acid, hexyl acrylate, and water was investigated. Experiments were performed in a batch reactor at temperatures of 338, 348, and 358 K catalyzed by sulfuric acid. Equilibrium constants decreased as temperature was decreased because of the exothermicity of the reaction. The heats of reaction and formation of hexyl acrylate were calculated as −38.6 and −449.59 kJ/mol from equilibrium experiments. Also theoretical calculation of heat of formation of hexyl acrylate was obtained through the application of three group/bond contribution methods, Joback, Benson bond, and Benson group. Compared to the experimental data, the Joback method provided more accurate results and its results are closer to the experimental values than those of Benson bond and Benson group contribution methods.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: 90 232 311 1493; 90 232 388 7776. E-mail: emine.
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by EBILTEM (2012 BIL 022) and TUBITAK (110 M 462) Scientific Research Projects. The kind help of Professor Ferhan S. Atalay during this study is also acknowledged.
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REFERENCES
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